Interspinous Spacer

ABSTRACT

An interspinous spacer implant includes first, second, third, and fourth arms that are divided into pairs that are relatively rotatable. In an insertion configuration, the first and third arms are disposed close together, and the second and fourth arms are disposed close together. Relatively rotating the arm pairs results in a deployed configuration where the first and fourth arms are disposed close together and the second and third arms are disposed close together, with each set of arms forming a respective saddle for receiving a corresponding spinous process. Related methods are also described.

This application is a divisional of prior application 10/851,889, filed May 21, 2004, the entirety of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to devices for treating spinal stenosis, and more particularly to interspinous spacers that can be implanted in a minimally invasive manner to treat spinal stenosis.

BACKGROUND OF THE INVENTION

Lumbar spinal stenosis (“LSS”), and sometimes called sciatica) is a condition of the spine characterized by a narrowing of the lumbar spinal canal. With spinal stenosis, the spinal canal narrows and pinches the spinal cord and nerves, causing pain in the back and legs. It is estimated that approximately 5 in 10,000 people develop LSS each year. For patients who seek the aid of a physician specialist for back pain, approximately 12-15% are diagnosed as having LSS.

Several causes of spinal stenosis have been identified, including aging, heredity, arthritis, and changes in blood flow to the lower spine. Aging is believed to be the most common cause, because as a person ages the ligaments connecting the bones of the spine can thicken and spurs may develop on the bones and into the spinal canal. The cushioning discs between the vertebrae also frequently deteriorate, and the facet joints may begin to break down. Heredity is believed to play a role in some cases because it may cause some people to have a smaller than average spinal canal, typically leading to LSS symptoms even at a relatively young age.

The most common symptoms of spinal stenosis are pain and difficulty when walking, although numbness, tingling, hot or cold feelings in the legs, and weakness or tiredness may also be experienced. In extreme cases spinal stenosis can cause cauda equina syndrome, a syndrome characterized by neuromuscular dysfunction that may result in permanent nerve damage.

Common treatments for LSS include physical therapy (including changes in posture), medication, and occasionally surgery. Changes in posture and physical therapy may be effective in flexing the spine to enlarge the space available to the spinal cord and nerves—thus relieving pressure on pinched nerves. Medications such as NSAIDS and other anti-inflammatory medications are often used to alleviate pain, although they are not typically effective at addressing the cause of the pain. Surgical treatments are more aggressive than medication or physical therapy, but in appropriate cases surgery may be the best way to achieve a lessening of the symptoms associated with LSS.

The most common surgery for treating LSS is decompressive laminectomy, in which the lamina of one or more vertebrae is removed to create more space for the nerves. The intervertebral disc may also be removed, and the vertebrae may be fused to strengthen unstable segments. The success rate of decompressive laminectomy has been reported to be in excess of 65%, with a significant reduction in LSS symptoms being achieved in many cases.

More recently, a second surgical technique has been developed in which the vertebrae are distracted and an interspinous spacer is implanted to maintain the desired separation between the segments. This technique is somewhat less invasive than decompressive laminectomy, but may provide significant benefits to patients experiencing LSS symptoms.

As with other surgeries, one consideration when performing surgery to implant an interspinous spacer is the size of the incision that is required to allow introduction of the device. Minimally invasive techniques are generally preferred, but the interspinous spacers previously known to the art did not work well with minimally invasive surgical techniques. The implantation profile presented by known spacers precludes introduction through a very small incision.

A need therefore exists for an interspinous spacer that can be implanted using minimally invasive surgical techniques. The present invention addresses that need.

SUMMARY OF THE INVENTION

Briefly describing one aspect of the present invention, there is provided an interspinous spacer that is configurable to a first, collapsed configuration, and a second, expanded configuration. The spacer may be implanted in a minimally invasive manner due to the reduced profile of the collapsed configuration of the spacer.

The present invention also provides a method of introducing an interspinous spacer between adjacent spinous processes. The method preferably comprises: (a) providing a spacer that is configurable to a collapsed configuration and to an expanded configuration; wherein said collapsed configuration presents an implantation profile that is at least 10% smaller than the corresponding profile when the spacer is in its expanded configuration; (b) causing said spacer to assume its collapsed configuration; (c) introducing said spacer into a medical patient while the spacer is in its collapsed configuration; and (d) allowing the spacer to assume its expanded configuration while in the medical patient. At the conclusion of the method the expanded-configuration spacer is positioned between adjacent spinous processes.

In one aspect of the invention the spacer comprises a blocking member and four arms extending therefrom. Accordingly, the spacer may have an “H”-shaped configuration when in a relaxed configuration, and an “I”-shaped configuration when in a collapsed configuration. The method of implanting such a spacer may comprise: (a) collapsing the spacer to its “I”-shaped configuration; (b) putting the collapsed spacer in a cannula to facilitate implantation in a medical patient; (c) from an oblique posterior approach positioning the distal end of the cannula in a medical patient so that the end of the cannula clears each of a pair of adjacent spinal processes; (d) pushing the collapsed spacer through the cannula until two of the arms exit the cannula and position themselves longitudinally beside the adjacent spinal processes; and (f) withdrawing said cannula while allowing or causing the spacer to continue through the cannula such that the spacer exits the cannula and the remaining two arms are positioned longitudinally on the other side of the adjacent spinal processes.

Objects and advantages of these and other aspects of the claimed invention will be apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C show an interspinous spacer according to one preferred embodiment of the present invention.

FIGS. 2A-2E show the interspinous spacer of FIG. 1 being implanted in a medical patient.

FIGS. 3A-3C show an interspinous spacer according to another preferred embodiment of the present invention.

FIGS. 4A-4D show an interspinous spacer according to another preferred embodiment of the present invention.

FIG. 5 shows an interspinous spacer according to another preferred embodiment of the present invention, including a rigid spacer portion to give the device an adjustable height.

FIG. 6 shows an interspinous spacer implanted in a medical patient.

FIGS. 7A-7L show alternative shapes of an interspinous spacer according to other preferred embodiments of the present invention.

FIGS. 8A-8M show representative configurations of an interspinous spacer according to other preferred embodiments of the present invention.

FIGS. 9A-9B show the use of a spacer/stabilizer, according to one preferred embodiment of the present invention.

FIGS. 10A-10K show alternative spacers/stabilizers, according to other preferred embodiments of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to certain preferred embodiments and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the preferred embodiments being contemplated as would normally occur to one skilled in the art to which the invention relates.

As indicated above, one aspect of the present invention relates to a method of providing an interspinous spacer between adjacent spinous processes. The method may be accomplished by: (a) providing a spacer that is configurable to a collapsed configuration and to an expanded configuration; wherein the collapsed configuration presents an implantation profile that is at least 10% smaller than the corresponding profile when the spacer is in its expanded configuration; (b) causing the spacer to assume its collapsed configuration; (c) introducing the spacer into a medical patient while the spacer is in its collapsed configuration; and (d) allowing the spacer to assume its expanded configuration while in the medical patient. At the conclusion of the method the expanded-configuration spacer is positioned between adjacent spinous processes.

As to the characteristics of the spacer generally, the spacer is designed to maintain a minimal distance between the spinous processes of adjacent vertebrae. As such, the spacer typically has a blocking portion that keeps the vertebrae from coming together. In general, the blocking portion maintains a distance of ¼″ to ½″ between the spinous processes.

Additionally, the spacer is preferably designed to fit snugly around the spinous processes, and thus to avoid being dislodged by movement of the spine. In one embodiment the spacer accomplishes that end by including “arms” extending from the blocking portion upward along both sides of the upper spinous process, and “arms” extending from the blocking portion downward along both sides of the lower spinous process. The arms keep the spacer from moving laterally with respect to the spinous processes. In some embodiments the arms have a relaxed configuration such that the distance between opposing arms is slightly less than width of a spinous process at that point. Thus, the arms will grip the spinous process to provide additional stability to the implanted spacer.

In one aspect of the invention the spacer comprises a blocking member with four arms extending therefrom. Accordingly, the spacer may have an “H”-shaped configuration when in a relaxed configuration, and an “I”-shaped configuration when in a collapsed configuration. The method of implanting such a spacer may comprise: (a) collapsing the spacer to its “I”-shaped configuration; (b) putting the collapsed spacer in a cannula to facilitate implantation in a medical patient; (c) from an oblique posterior approach positioning the distal end of the cannula in a medical patient so that the end of the cannula clears each of a pair of adjacent spinal processes; (d) pushing the collapsed spacer through the cannula until two of the arms exit the cannula and position themselves longitudinally beside the adjacent spinal processes; and (f) withdrawing said cannula while allowing or causing the spacer to continue through the cannula such that the spacer exits the cannula and the remaining two arms are positioned longitudinally on the other side of the adjacent spinal processes.

In one embodiment the spacer is collapsible by virtue of the fact that the material used to make the spacer is very elastic and pliable. In such embodiments the spacer arms may be manipulated so as to transform the H-shaped configuration to an I-shaped configuration merely by bending the arms from an orientation that is generally perpendicular to the crossbar of the “H” to an orientation that is generally parallel to the crossbar of the “H.” Accordingly, in one embodiment the H-shaped implant is converted to an I-shaped implant by folding the upwardly and downwardly extending arms so that they extend horizontally, i.e., the folded arms extend in a direction that is generally parallel to the crossbar of the “H.” When the force manipulating the spacer arms is released, the arms then return to their original orientation that is generally perpendicular to the crossbar of the “H.” FIGS. 1A-1C, described below, show the manipulation of one H-shaped embodiment.

In another embodiment the spacer is collapsible by virtue of a pivot point near the center of the spacer. Such embodiments may work much like a pair of scissors, with four arms extending from a central pivot. As with scissors, the device may be converted from a generally “X”-shaped device to a generally “I”-shaped device by pivoting one pair of arms relative to the other. FIGS. 4A-4C, described below, show one such pivoting embodiment.

The ability of the spacer to assume a collapsed configuration allows the spacer to be implanted using a minimally invasive surgical technique. Most preferably, the surgery is accomplished using a posterior oblique approach through a small incision in the patient's back.

Regardless of the surgical approach used for implantation, when the spacer passes into the body it presents an “implantation profile” corresponding to the size of the implant as it passes through the plane of the opening in the body. The implantation profile therefore defines the size of the opening required to accept the implant. While it is appreciated that different surgeons may use different orientations of a spacer when implanting it into a patient, there is generally one orientation that presents a smaller implantation profile than the others. For the purposes of this disclosure then, the term implantation profile is used to identify the size of an implant as it passes through an opening in the body, given that the implant is manipulated so as to present the smallest possible implantation profile. To the extent the size of the portion of the implant that is passing through the opening increases or decreases as different portions of the implant pass through the opening, the implantation profile is the maximum size presented to the opening during implantation, and therefore corresponds to the minimum opening size required to accommodate the implant.

In one embodiment of the present invention the implantation profile is at least about 10% smaller than the corresponding profile when the spacer is in its expanded configuration. In other embodiments the implantation profile is at least about 20% smaller than the corresponding profile when the spacer is in its expanded configuration. More preferably, the implantation profile is about 25% smaller than the corresponding profile when the spacer is in its expanded configuration. Most preferably, the implantation profile is at least 50% smaller than the corresponding profile when the spacer is in its expanded configuration.

An interspinous spacer for use in the invention may be formed from a wide variety of biocompatible materials that can undergo reversible elastic deformation. Examples of such materials include elastic or rubbery polymers, hydrogels or other hydrophilic polymers, or composites thereof. Particularly suitable elastomers include silicone, polyurethane, copolymers of silicone and polyurethane, polyolefins, such as polyisobutylene and polyisoprene, neoprene, nitrile, vulcanized rubber and combinations thereof.

Examples of polyurethanes include thermoplastic polyurethanes, aliphatic polyurethanes, segmented polyurethanes, hydrophilic polyurethanes, polyether-urethane, polycarbonate-urethane and silicone polyetherurethane. Other suitable hydrophilic polymers include polyvinyl alcohol hydrogel, polyacrylamide hydrogel, polyacrylic hydrogel, poly(N-vinyl-2-pyrrolidone hydrogel, polyhydroxyethyl methacrylate hydrogel, and naturally occurring materials such as collagen and polysaccharides, such as hyaluronic acid and cross-linked carboxyl-containing polysaccharides, and combinations thereof.

In other embodiments the spacer is made of a metal that can undergo reversible elastic deformation, such as shape memory metals or nickel titanium.

The nature of the materials employed to form the blocking portion of the spacer should be selected so the formed implants have sufficient load bearing capacity. In preferred embodiments, a compressive modulus of at least about 0.1 Mpa is desired, although compressive strengths in the range of about 1 Mpa to about 20 Mpa are more preferred. Most preferably the compressive modulus is at least about 5 Mpa.

In some embodiments the spacer may also advantageously deliver desired pharmacological agents. The pharmacological agent may be a growth factor that may advantageously repair damaged tissue or bone, and may include an osteoinductive factor (e.g., a bone morphogenetic protein), transforming growth factor-ss (TGF-ss), insulin-like growth factor, platelet derived growth factor, fibroblast growth factor or other similar growth factor or combination thereof having the ability to repair tissue or bone.

In other forms of the invention, the spacer may comprise a pharmacological agent used for treating various spinal conditions, including degenerative disc disease, spinal arthritis, spinal infection, spinal tumor and osteoporosis. Such agents include antibiotics, analgesics, anti-inflammatory drugs, including steroids, and combinations thereof. Other such agents are well known to the skilled artisan. These agents are also used in therapeutically effective amounts. Such amounts may be determined by the skilled artisan depending on the specific case.

The pharmacological agents, if any, are preferably dispersed within the spacer for in vivo release. The pharmacological agents may be dispersed in the spacer by adding the agents to the spacer when it is formed, by soaking a formed spacer in an appropriate solution containing the agent, or by other appropriate methods known to the skilled artisan. In other forms of the invention, the pharmacological agents may be chemically or otherwise associated with the spacer. For example, the agents may be chemically attached to the outer surface of the spacer.

In some embodiments the device may include one of more X-ray markers such as tantalum markers to assist in positioning the implant. A combination of larger x-ray markers and smaller x-ray markers may be used to facilitate observing the orientation of the device when implanted. The x-ray markers can be more readily observed on x-rays, making the positioning and orientation of the device more easily observed and corrected.

Referring now to the drawings, FIGS. 1A-C show an interspinous spacer according to one embodiment of the present invention. Spacer 10 includes a blocking member 15 and arms 11, 12, 13, and 14. When the spacer is in its relaxed (expanded) configuration as shown in FIG. 1A, it resembles an “H,” with arms 11, 12, 13, and 14 being the legs of the H, and blocking member 15 being the crossbar. As illustrated in the drawing, the arms are generally perpendicular to the blocking member when the spacer is in its relaxed/expanded configuration.

To use the spacer, the arms are manipulated to be parallel to the blocking member, manipulating the spacer to its collapsed (implantable) configuration as illustrated in FIGS. 1B and 1C. The manipulation makes the spacer assume the shape of an “I” rather than the shape of an “H.” Arrows a, b, c, and d, show the direction of the manipulation to transform the “H” to an “I.” As indicated above, the preferred manipulation converts the H-shaped implant to an I-shaped implant by folding the upwardly and downwardly extending arms so that they extend horizontally in a direction that is generally parallel to the crossbar of the “H.” When the spacer is manipulated to its collapsed/implantable configuration, the implantation profile of the profile is reduced.

FIGS. 2A-E show one embodiment of a method for implanting the spacer. In FIG. 2A, spacer 10 is loaded in cannula 20 while the spacer is in its collapsed/implantable configuration. The spacer is in its collapsed configuration so that its implantation profile is reduced from the corresponding profile when the spacer is in its relaxed configuration.

Cannula 20 is positioned between two spinous processes, with the tip 20 a of the cannula extending just beyond the spinous processes when the cannula is inserted from a posterior oblique approach. When the cannula is positioned, the spacer is pushed from the cannula so that the leading pair of arms 22 and 24 begins to unfold from its collapsed/implantable configuration to its relaxed/expanded configuration, as shown in FIG. 2B. As the arms unfold they extend upward and downward along one side of two spinous processes, as shown in FIG. 2C.

The cannula is then withdrawn as the spacer is ejected, as shown in FIG. 2D. The blocking portion 25 of spacer 10 is positioned between the two spinous processes, and the second pair of arms 21 and 23 unfolds to extend upward and downward along the second side of the spinous processes, as shown in FIG. 2E.

In another embodiment the spacer may have indents and/or other surface features to facilitate collapsing and implanting the spacer, or to avoid cracking or tearing the implant when the arms are folded to their collapsed configuration. Features such as ridges to facilitate gripping the spinous processes may also be included.

For example, FIGS. 7A through 7L show embodiments having surface features to reduce compressive forces on the outside walls during deformation (in the “I” shape). The illustrated surface features, which are merely examples of the many types and/or shapes of surface features that may be utilized, act to reduce compressive forces on the outside surface of the implant when the implant is folded from its “H” configuration to its “I” configuration.

In addition or as an alternative, surface features may be included on the “inner” surface of the implant to reduce tensile forces on those surfaces when the implant is deformed. FIGS. 8A through 8M show some preferred embodiments of such surface features. Here too, the illustrated surface features are merely examples of the many types and/or shapes of surface features that may be utilized to reduce “stretching” or tensile forces on the inside surface of the implant when the implant is folded from its “H” configuration to its “I” configuration.

One embodiment effective to reduce both compressive and tensile forces is shown in FIGS. 3A-3C. In that embodiment, spacer 30 comprises arms 31, 32, 33, and 34, and blocking portion 35. Blocking portion 35 includes at least one indent 35 a, and may include two indents as shown in FIG. 3A. The implant is manipulated from its relaxed configuration to its straightened configuration as before. Arms 31 and 32 are folded downward until they are generally horizontal and lie in the same direction as blocking member 35. Arms 32 and 34 are folded upward until they are generally horizontal and lie in the same direction as blocking member 35. The folded implant can then be placed in a cannula and pushed through a small opening in a patient's body as described above. Once implanted, the device relaxes to its H-shaped configuration with indents 35 a centering the implant around the spinous processes and arms 31, 32, 33, and 34 preventing lateral displacement.

In another embodiment the implant may have one or more arms that pivot in relation to other non-pivoting arms. The arms preferable pivot around a central point in the blocking member. Most preferably, the device comprises four arms arranged as two pivoting pairs, with each of the two pairs of arms pivoting together. In the most preferred embodiments, the pivoting arms are substantially rigid, although they may be elastic in other embodiments.

FIG. 4A shows one preferred embodiment of the present invention in which the implant has pivoting arms. Pivot post 47 defines the point around which arms 41, 42, 43, and 44 pivot. In the illustrated embodiment, arms 41 and 42 form one pair, and arms 43 and 44 form another pair. A spring 48 may be used to bias the arms to their closed position, as shown in FIGS. 4A and 4B. In some embodiments spring 48 is wound at least partially around pivot post 47.

To operate implant 40, the implant is preferably allowed to adopt its closed position as shown in FIG. 4A. In this position the implant has its minimum implantation profile, allowing the closed implant to pass through a small incision in a patient. After the implant has been introduced into the patient, the implant is opened by allowing the arms to move in the direction of the arrows shown in FIG. 4C. This allows the implant to adopt its open configuration as shown in FIG. 4D. In that configuration, the implant has a profile that is larger than the profile of the implant in its closed configuration.

In some embodiments the blocking member portion of implant 40 has a concave shape when the arms are opened to their open configuration. This allows the blocking member to fit more securely around the interspinous processes.

In another embodiment of the present invention the implant includes a spacer portion between the two pair of opposing arms. The spacer portion may give the device an adjustable height, with varying sizes of rigid spacer portions being available.

FIG. 5 shows one embodiment of the device of the present invention having a spacer/stabilizer portion. In spacer 50, arms 51, 52, 53, and 54 extend from blocking portion 55 and spacer/stabilizer portion 56.

The device with a spacer is used in a manner similar to the device without a spacer. Accordingly, arms 51, 52, 53, and 54 may fold down to lie horizontally along the axis of blocking member 55 and spacer portion 56, so that the device has an implantation profile that is at least 10% smaller than the corresponding profile of the device in its relaxed configuration.

Alternative embodiments of an interspinous spacer having a spacer/stabilizer are shown in FIGS. 9A-9B, and in FIGS. 10A-10K. These embodiments are particularly effective for reducing or preventing in vivo deformation of the device, and thus for reducing or preventing dislocation and/or migration after implantation. In the embodiments illustrated in the drawings, the central shank 91 provides the spacing effect for varying desired thicknesses, while the end portions 92 and 93 provide stabilization against in vivo deformation into the “I” shape. As may be appreciated by persons skilled in the art, the illustrated spacers/stabilizers may be incorporated into the implant in vivo, with the spacer/stabilizer being installed and assembled only after the device has assumed its “H” shape.

FIG. 6 shows an interspinous spacer according to one preferred embodiment of the present invention, after implantation in a medical patient. Arms 61, 62, 63, and 64 of spacer 60 grip the spinous processes 66 a and 66 b to hold the spacer in position.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. 

1. A spinal implant comprising: a first arm assembly comprising first and second arms and an intermediate section disposed between and connecting the first and second arms; the first and second arms extending in generally opposite directions and the intermediate section disposed generally transverse to the first and second arms; a second arm assembly comprising third and fourth arms and an intermediate section disposed between and connecting the third and fourth arms; the third and fourth arms extending in generally opposite directions and the intermediate section disposed generally transverse to the third and fourth arms; the implant movable from a first insertion configuration to a second deployed configuration by pivoting the second arm assembly relative to the first arm assembly about a pivot axis; wherein, in the first configuration: the first and third arms extend in the same general direction in spaced relation to each other; the second and fourth arms extend in the same general direction in spaced relation to each other; a distal tip of the first arm is disposed closer to the third arm than the fourth arm; a distal tip of the second arm is disposed closer to the fourth arm than the third arm; the intermediate sections of the first and second arm assemblies overlap when viewed along the pivot axis; wherein, in the second configuration: the first and fourth arms extend in the same general direction and form a lower saddle for receiving a first spinous process of a pair of adjacent spinous processes; the second and third arms extend in the same general direction and form an upper saddle for receiving a second spinous process of the pair of adjacent spinous processes; a distal tip of the first arm is disposed closer to the fourth arm than the third arm; a distal tip of the second arm is disposed closer to the third arm than the fourth arm; the intermediate sections of the first and second arm assemblies overlap when viewed along the pivot axis.
 2. The spinal implant of claim 1 further comprising a biasing element for biasing the first arm assembly in a first rotational direction relative to the second arm assembly.
 3. The spinal implant of claim 1 wherein, in the second configuration, the first and fourth arms angle toward each other.
 4. The spinal implant of claim 1 further comprising a pivot pin disposed along the pivot axis.
 5. The spinal implant of claim 1 wherein the first arm, the second arm, and the intermediate section of the first arm assembly are substantially rigid.
 6. The spinal implant of claim 1 wherein the first and second arms have substantially the same relative orientation in the first and second configurations; and the third and fourth arms have substantially the same relative orientation in the first and second configurations.
 7. A spinal surgical implantation method comprising: providing an interspinous spacer implant comprising: a first arm assembly comprising first and second arms and an intermediate section disposed between and connecting the first and second arms; the first and second arms extending in generally opposite directions and the intermediate section disposed generally transverse to the first and second arms; a second arm assembly comprising third and fourth arms and an intermediate section disposed between and connecting the third and fourth arms; the third and fourth arms extending in generally opposite directions and the intermediate section disposed generally transverse to the third and fourth arms; inserting the implant between first and second adjacent spinous processes in a first insertion configuration wherein: the first and third arms extend in the same general direction; a distal tip of the first arm is disposed closer to the third arm than the fourth arm; a distal tip of the second arm is disposed closer to the fourth arm than the third arm; while the implant is disposed between the adjacent spinous processes, changing the implant from the first configuration to a second deployed configuration by pivoting the first arm assembly relative to the second arm assembly about a pivot axis; wherein, in the second configuration: the first and fourth arms extend inferiorly and form a lower saddle that receives an inferior spinous process of the adjacent spinous processes; the second and third arms extend superiorly and form an upper saddle that receives a superior spinous process of the adjacent spinous processes; the intermediate sections of the first and second arm assemblies overlap when viewed along the pivot axis.
 8. The method of claim 7 wherein, after the changing the implant to the second configuration, the first and third arms are disposed on a first lateral side of the adjacent spinous processes and the second and fourth arms are disposed on a second opposite lateral side of the adjacent spinous processes.
 9. The method of claim 7 wherein, after the changing the implant to the second configuration: a distal tip of the first arm is disposed closer to the fourth arm than the third arm; a distal tip of the second arm is disposed closer to the third arm than the fourth arm.
 10. The method of claim 7 further comprising biasing the first arm assembly in a first rotational direction relative to the second arm assembly.
 11. The method of claim 7 further comprising introducing the spacer implant into a patient while the spacer implant is in the first configuration.
 12. The method of claim 7 wherein the changing the implant from the first configuration to the second configuration comprises disposing the intermediate sections of the first and second arm assemblies so as to extend through a sagittal plane defined by the adjacent spinous processes. 